To understand how murine leukemia viruses cause lymphatic leukemia, recombinants have been isolated between a virus that causes B cell leukemia and viruses that cause T cell leukemia. Using fluorescent-labeled antibody against viral and lymphoid differentiation antigens and the fluorescence activated cell sorter, we will identify lymphoid subpopulations susceptible to infection and/or leukemogenesis by each parental or recombinant virus in order to map the region of viral genome responsible for transformation of specific lymphoid target cells.